Recent outbreaks of highly pathogenic influenza virus have highlighted the need for a better understanding of the molecular mechanisms of influenza virus pathogenesis and the host immune response to these viruses. Viral infection triggers a prompt anti-viral response, including the release of proinflammatory cytokines from cells such as macrophages and dendritic cells. Type I interferons (IFNs) coordinate the host response by upregulating a large number of IFN stimulated genes (ISGs). Several ISGs have direct antiviral activity, while others impact upon the antiviral response by modulating the immune system. We have recently shown that one of these ISGs, ISG15, functions as a critical IFN induced anti-viral molecule. ISG15 is an ubiquitin homolog that is strongly upregulated by IFNs, toll receptor ligation, and viral infection. ISG15 deficient mice display increased lethality following infection with several viruses, including both influenza A and B viruses. Yet the mechanism by which ISG15 exerts this antiviral activity is unknown. ISG15 conjugates to a wide array of intracellular proteins, targeting numerous biological processes. Human ISG15 is also released from cells and functions as a cytokine, activating immune cells including dendritic cells. We have shown that viral infection induces both ISG15 conjugate formation and its release into the serum. We also see a potent proinflammatory cytokine response in ISG15-/- mice following viral infection. Together, these data lead us to the hypothesis that ISG15 regulates the function of dendritic cells during viral infection to modulate the innate, and potentially the adaptive immune response. The goal of this proposal is to test this underlying hypothesis via the follow Aims. The studies in Aim 1 will determine if ISG15 deficient dendritic cells have an intrinsic defect in their response to viral infection. We will evaluate their ability to produce proinflammatory cytokines and function as antigen presenting cells. The studies in Aim 2 will determine if the immune response to viral infection in altered in ISG15 deficient mice. Finally, in Aim 3 we will test the hypothesis that expression of ISG15 in dendritic cells will rescue ISG15-/- mice from lethality. The results obtained from these studies will provide important insight into a potential mechanism of action for a newly identified antiviral molecule, and provide the necessary groundwork for future investigations into ISG15 activity. [unreadable] [unreadable] [unreadable]